The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In the current network application, the position of multimedia services, such as video, becomes increasingly important, and the multicast technology is an important bearer technology of multicast technology for network multimedia services. The multicast is a one-to-many or many-to-many multiparty communication mode. In the multicast model as shown in FIG. 1, the path through which a video stream passes in a network forms a tree structure, the same video stream transfers on each branch of the tree, and the video stream is transferred to all the receivers by copying the video stream at the bifurcations of the tree. This tree is referred to as a distribution tree. In the multicast routing protocol, a distribution tree is formed firstly, and a video stream is transferred from a server to all the receivers by transferring the video stream on each branch of the distribution tree and copying the video stream at the bifurcations of the tree.
With the IPTV service as an example, if a user wants to watch a TV program on a channel, the user first needs to send a packet for joining a multicast group, which is referred to as a join packet for short below, to a nearest layer 3 network device, and the layer 3 network device establishes a corresponding routing forwarding state. Because the routing forwarding state can be used for instructing the forward of a protocol packet and a multicast data packet, the routing forwarding state is generally referred to as multicast forwarding instruction information. In addition, an outgoing-interface of the multicast data packet further needs to be added in the routing forwarding table item, and a join packet is sent to an upstream layer 3 network device in the direction of the multicast source server to which the TV program of the channel pertains. A similar action is performed on the upstream layer 3 network device. In other words, a routing forwarding state is established, an outgoing-interface is added to a routing forwarding table item, and then a join packet is sent to an upstream side of the upstream layer 3 network device. After the root node of the multicast data packet distribution tree receives the join packet, the layer 3 network device of the root node pushes the multicast data packet along the same distribution tree to the layer 3 network device nearest to the user.
The join packet sent by the user includes a multicast source group address pair, i.e. an address of a program of a channel, and a multicast session is identified by the multicast source group address pair. The address pair is generally represented in the form of (S,G) or (*,G), or uniformly represented by (S/*,G). The S represents an address of a designated multicast source, in other words, the S represents the IP address of the source which is sent to the multicast group G. The G represents an address of a designated multicast group for identifying the collection of all the receivers in a multipoint communication service, i.e. a multicast group. For example, all the receivers of an IPTV program belong to the same multicast group. The * represents any multicast source, that is, no multicast source is designated by the receiver. A multicast source group address pair (S/*,G) corresponds to a group of states in a layer 3 network device on the distribution tree, and the layer 3 network device on the distribution tree establishes a corresponding routing forwarding table item for the multicast source group. The message carrying the state is referred to as a protocol packet.